It's been a while, but I believe more of the Sevii Islands become available, where there are a handful of Gen 2/3 Pokémon like Phanpy, Larvitar and Unown, can't remember them all. Moltres can be caught on One Island, and after finishing the post-game, one of the legendary beasts starts roaming. I'm uncertain about evolutions added post-Gen-1.

Note: Espeon and Umbreon are always unobtainable in FR/LG, even after obtaining the national Pokedex, due to the removal of the in-game clock.
Note 2: this is for LeafGreen only. Version-exclusive Pokémon for FR/LG can be found here.

Source: looked through my own copy of LeafGreen, and consulted this to make sure I wasn't missing any trade evolutions (since I have no way to trade with this game).